1. Field of the Invention
This invention relates to a structure of a piezoelectric film which composes a piezoelectric film element. More particularly, this invention relates to a structure of a piezoelectric film element which is preferable for a drive source for discharging ink for an ink-jet recording head.
2. Description of the Related Art
An ink-jet recording head of an on-demand type comprises a piezoelectric film element which functions as a drive source for discharging ink. This piezoelectric film element comprises a piezoelectric film made of piezoelectric ceramics, and an upper electrode and a lower electrode to hold the piezoelectric film in between. The application of a desired electric field to the piezoelectric film element causes changes in volume, while the application of pressure causes changes in voltages. Since many piezoelectric ceramics with a perovskite crystal structure remarkably exhibit the above-described action, such piezoelectric ceramics are used as materials for the piezoelectric film. The piezoelectric film element provided with such a crystal structure is disclosed in, for example, Applied Physics Letters, 1991, Vol. 58, No. 11, p.p. 1161-1163. A prior art example of the ink-jet recording head using the piezoelectric film element is disclosed in, for example, a specification of U.S. patent application Ser. No. 5,265,315.
Since the ink-jet recording head in these days are being required to achieve higher precision printing, the volume of a pressure room is getting smaller. In order to make the pressure room with the small volume discharge ink in an appropriate amount, it is necessary to cause higher pressure in the piezoelectric film element. This pressure is generated as accumulation of very little strains in individual crystal structures. Accordingly, it is supposed that as the thickness of the piezoelectric film is increased, higher pressure can be obtained. Moreover, by increasing the thickness of the piezoelectric film, it is possible to prevent the lowering of piezoelectric properties due to the generation of a strong electric field in the piezoelectric film. Therefore, attempts are being made to increase the thickness of the piezoelectric film by various film forming methods.
A piezoelectric film which is generally used has: a two-component composition containing lead zirconate titanate (hereinafter sometimes referred to as "PZT") as a principal component; or a three-component composition prepared by adding a third component of PZT to the two-component composition. As a method for forming the piezoelectric film, for example, "JOURNAL OF APPLIED PHYSICS" (Vol. 83, Number 4, Feb. 15, 1998, p.p. 2202-2208) discloses the technique to form, by a sol-gel method, a film of Pb(Zr.sub.0.30 Ti.sub.0.70)O.sub.3 (PZT30/70) over an electrode made of platinum/titanium in the temperature environment of 510.degree. C.
This sol-gel method is the method of giving dehydration treatment to a hydrate complex (sol) of a hydroxide of a metal component of a PZT type piezoelectric film to turn it into a gel and of heating and burning the gel to adjust an inorganic oxide (piezoelectric film). This method makes it possible to form the film by repeating the coating, drying and pyrolyzing of a precursor of the PZT type piezoelectric film for several times until a specified thickness is obtained. Accordingly, this method is excellent in the composition control and is preferred for the adjustment of the thickness of the piezoelectric film. Moreover, patterning using a photoetching step is also possible and has actually been applied to an ink-jet recording head.
For example, when a PZT film with a thickness of about 0.4 .mu.m is to be formed by the sol-gel method, the step of spin coating, drying and pyrolyzing a sol for the PZT film is repeated for several times (for example, four times) and the step of RTA thermal treatment (final annealing) is then taken, thereby obtaining the desired PZT film.
However, the inventors of this invention have found that if the piezoelectric film is formed by the above-described sol-gel method and if an attempt is made to increase the film thickness to a certain degree, residual stress affects the inside of the piezoelectric film and cracks may be sometimes generated on the surface of the film when crystal grains are caused to grow in the pyrolyzing step and the RTA step. It is assumed that this phenomenon is caused because heat stresses act upon each other in a complicated manner when a molecular structure which is in the amorphous state in the pyrolyzing step or the RTA step of the piezoelectric film is turned into a minute crystal structure. Accordingly, it has been impossible to form a piezoelectric film with a large film thickness beyond a certain degree and technical limitations have been imposed upon the achievement of high precision of the ink-jet recording head.